Nozzles arranged in spray guns have proven successful in lacquer spraying methods for so-called two-component lacquers. The exactly metered mixing of the components (even at the beginning and end of the mixing process) is not particularly critical in such lacquer spraying devices.
Such nozzles have also been used for the production of solid or porous plastics. The known nozzles are adequate for simple applications such as polyurethane insulating foam technology. For the production of high quality plastic components by the so-called RIM technique the use of the known nozzles is, for various reasons, subject to many restrictions.
German patent No. 1,251,190 describes a two component spray gun (in particular for spraying epoxy resins) in which two coaxial liquid nozzles open into a widening and then conically tapered mix chamber. An attachment having a mix chamber in which a cone points towards the nozzle openings is arranged on the end face having the coaxial nozzle openings. Compressed air nozzles are arranged inside and outside, with which air is blown directly into the mixture and influences the issuing stream of mixture. Metered mixing of the components is not guaranteed at the beginning or the end of the spray process since the apparatus is actuated using a hand lever. This complicated mixing chamber can be cleaned only with rinsing agent.
The two component spray gun for polyester resin lacquers described in German Auslegeschrift No. 1,086 598 has similar disadvantages. In this case, the first and second nozzle are fixed in succession and can be closed by means of a common injection needle. This gun has the disadvantage that one component leads as soon as the nozzle is opened and lags when it is closed so that some parts of the components remain unmixed. The spray gun has the additional disadvantage that it needs to be cleaned with a rinsing agent.
In the method disclosed in German Offenlegungsschrift No. 2,252,008 (for combining components such as polyol and isocyanate), coaxial nozzles which open into a mixing chamber are also provided. The mixing chamber is enlarged or reduced by a spring-mounted insert, according to the pressure of a main component. The nozzle openings are opened and closed by the component pressures acting against compression springs. This method of control cannot meet current requirements due to the inaccuracy of the moment of opening and closing. The mixing chamber is completely displaced in the closed state by the insert. However, a film of mixture can remain between two radial surfaces and reacts to completion so that the mixing chamber clogs up over time. The main component passes through the control gap into the mixing chamber. In the present design, only very slow atomization rates can be obtained for this main component. This is due to the relatively large diameter of the control gap which has to be selected so that no particles are retained in this control gap since they could cause interruptions in operation.
Finally, German Auslegeschrift No. 2,031,739 (corresponding to U.S. Pat. No. 3,771,963) describes a process and an apparatus for the production of foams or homogeneous substances from at least two reactive components. The components are brought separately to the mixing zone and are combined just prior to injection into the mixing zone. The components are atomized together under elevated pressure and are mixed together in the mixing zone for a residence time of at least 1/1000 second. The mixture is then ejected. In the apparatus, the supply channels are combined just in front of the nozzle opening and end in a common nozzle which points into a mixing chamber. This apparatus also has a pressure control means operating against compression springs for the components with the known inaccuracies. Moreover, one component is supplied centrally through the injection needle and can be checked upstream by a spring-loaded pressure valve. Subsequent dripping is unavoidable and results in contamination and delay of this component. A further disadvantage lies in the fact that this nozzle is generally not suitable for use in the RIM process because the central supply bore would clog up.
All these previously known nozzle-type apparatuses thus have the following disadvantages: they have to be cleaned with rinsing agents; they operate too inaccurately at the beginning and end of the combining of the components; and, small quantities of the mixture cannot therefore be introduced into the mixing chamber.
The object of the present invention is to find a nozzle and a process for the operation thereof with which the combining of the components in a metered ratio takes place exactly even at the beginning and end (in other words one component does not lead or lag) and where operation is possible without rinsing agents and the mixing of small quantities of components is possible.